This proposal concerns the transcriptional organization and regulation of the ribosomal genes in the myxomycete Physarum polycephalum. We shall take advantage of the fact that large quantities of ribosomal DNA (rDNA) and its restriction fragments, as well as associated histones and nucleolar proteins, can be obtained from growing Physarum, where the ribosomal genes are active, and from microsclerotia, where ribosomal RNA synthesis is suppressed. We shall map in detail sequences in the gene and spacer regions of the palindromic rDNA molecule. Transcribed sequences of the rDNA will be identified, and sites of initiation of transcription will be localized. Proteins associated with rDNA sequences in native rDNA chromatin will be characterized. Transcription of the rDNA molecule will be studied in vitro in reconstituted systems employing homologous RNA polymerase as well as in vivo using microinjection techniques. We shall pursue research on the localization of nucleosomes on gene and spacer sequences of the rDNA and on the structure of nucleoprotein subunits comprising transcribing rDNA chromatin.